1	Astro 1050 Mon. Sep. 8, 2003 How the stars move across the sky during the night The celestial sphere Cycles Motion of the Sun Seasons The Moon on Wednesday: phases and eclipses
2	The View From A Rotating Platform Imagine being in a rotating restaurant on top of a tall building. All the outside objects are very far away – much farther than the distance across the room. Paint the view on the windows – and keep the people near the center of the room – away from the windows themselves. Can the people tell if the room is rotating, or if the painted windows are just moving around the room? Which is more reasonable – a rotating room or rotating painted windows?
3	The “Celestial Sphere” If you “paint” the stars on a sphere much bigger than the earth, then you can obtain the motion of the stars by pretending that sphere rotates, rather than the earth. For most people that motion is easier to “see”. The sphere rotates once every day (actually once every 23^h 56^m for reasons we’ll see later) We will see later that the sun, moon, and planets move slowly along the sphere relative to the stars. You can think of them (for a night or so) as “stars” which move almost with the other fixed – but drift relative to them from night to night. The celestial sphere will also have marked on it a projection of the earth’s latitude and longitude system, as well as a few other special points and circles.
4	Celestial Sphere
5	Nomenclature HORIZON: The horizontal circle which separates the part of the sky visible to you and the part of the sky hidden by the earth. ZENITH: The point on the sky directly overhead. MERIDIAN: The circle which starts on the northern horizon, runs through the zenith, continuing on to the southern horizon. It separates the eastern half of the sky from the western half. CELESTIAL POLES: The points where the extension of the rotation axis of the earth would intersect the celestial sphere. CELESTIAL EQUATOR: The circle around the sky which would be a projection of the earth’s equator.
6	Limiting Cases At the Earth’s north pole, looking overhead all stars appear to circle around the north celestial pole. At the equator: Stars on the celestial equator rise in the east, move overhead, then set in the west The N and S celestial poles are just on your N and S horizons, and stars near those points still circle around them. But those stars are only visible for the upper half of their circles.
7	Intermediate cases like Laramie Stars close enough to the north celestial pole are always above the horizon, and just circle the pole star. (CIRCUMPOLAR STARS) Stars on the celestial equator rise in the east, move higher along a slanted path which crosses the “meridian” to the south of the zenith, then descend and set due west. Stars far enough to the south never make it above the horizon.
8	Star Motion from the Northern Hemisphere
9	Precession of the Earth The earth’s axis of rotation is tilted 23.5⁰ relative to the plane containing the sun and other planets. The gravity from the Sun and moon is trying to tip the earth just like gravity is trying to tip a spinning top. As with the top, the axis of the earth wobbles or PRECESSES in space, with a 26,000 year period. Because the directions to the celestial poles are defined by the spin axis – those poles move with time. It isn’t that the stars move – it is that the grid we paint on the celestial sphere has to be redrawn from time-to-time. Eventually Polaris will not be the “pole” star.
10	Motion of the Sun throughout the year. Plot position of Sun relative to stars, over one full year. Similar to what we have been doing with moon Complicated by fact you can’t see Sun and stars at same time. Once you have full map of sky, you can work this out by seeing what stars are opposite sun 12 hours later.
11	Plotting the Ecliptic on the Celestial Sphere
12	Consider the Sun’s daily motion at different times of the year
13	How the Sun’s location affects the seasons:
14	Special Locations on the Earth
15	Why are the planets found near the ecliptic?
16	Superior vs. Inferior Planets
17	Inferior Planets
18	Effect of Elliptical Orbit on Climate Seasons almost entirely due to TILT of Earth Seasons opposite (not the same) in N & S Hemispheres Earth’s orbit slightly elliptical Slightly closer to the sun in N. Hemisphere Winter But this changes as tilt precesses in 26,000 yr cycle Expect N. Hemisphere winter to be slightly milder Positions of continents and oceans actually more important Effect is important for Mars -- more elliptical orbit Cyclic variations in climate as tilt precesses (and tilt and ellipticity also gets slightly larger and smaller VERY IMPORTANT TOPIC (Re: Global Warming)
19	For Wednesday: